There are known thermoplastic polyesters, which are widely utilized as films, fibers, supports for magnetic tapes, structural materials infields such as the electronic or the biomedical. They are generally prepared by polycondensation of aromatic diacides or their derivatives with diols. Typical examples are polyethyleneterephthalate and polybutyleneterephthalate, which are obtained by polycondensation of terephthalic acid or a of a derivative thereof, for example a diester, with ethylene glycol or butandiol, respectively.
There are also known thermoelastomeric polyesters obtained by condensation of a diacid or a derivative thereof, such as e.g. a diester, with a low molecular weight glycol and with a polyglycol, generally a polyoxyalkylene glycol having a molecular weight from 1000 to 3000.
The thermoplastic and thermoelastomeric polyesters are characterized by high mechanical and electrical characteristics, a good stability to solvents and to hydrolysis.
For a plurality of uses such as films, supports for magnetic tapes, fibers, in various sectors of goods, they exhibit however considerable drawbacks, as they have a low oil and water-repellency, a low resistance to soiling and low free flowing properties, characterized by a high friction coefficient, so that a subsequent finishing treatment of such articles is absolutely necessary.
In particular, as regards the use for magnetic recording tapes, where the material flows on metal or plastic surfaces, a surface treatment of said surface with a lubricant is required in order to reduce friction and to secure the sliding of the tape and preserve it as much as possible from the wearing. For these uses, very thin layers on the substrate surface, generally from 50 to 1,000 .ANG., are sufficient.
For other uses, for example in particular in the biomedical field, the conventional hydrogenated polyesters, although exhibiting - in comparison with other utilizable materials - improved impermeability and mechanical properties, do not exhibit good biocompatiblity and antithrombogenicity.
With a view to overcoming these drawbacks, the hydrogenated polyesters are therefore subjected to finishing processes or surface treatments.
These finishing processes or these surface treatments are generally carried out, for example, with a fluorinated diol or with a solution or a dispersion of a fluorinated plastic polymer, such as, for example, a polytetrafluoroethylene dispersion, depending on the type of application. However, these fluorinated coatings exhibit the drawback of not sufficiently adhering to the substrate, owing to the surface properties which are typical of the fluorinated polymer and which interfere with the adhesion. By consequence, since it is not possible to maintain the fluorinated layer adherent to the article for a sufficiently long period of time, a degradation of the surface properties occurs, such as resistance to chemical agents, oil- and water-repellency, resistance to soiling, free flowing which, conversely, are indispensable for a plurality of uses.
An alternative method of increasing the duration of the above said surface properties consists in chemically binding a fluorinated monomer to the polymeric substrate by grafting. This process can be carried out, for example, by using radiation or electric discharge.
With such method, however, the attainment of a uniform layer is strictly related to the substrate nature. In fact, on substrates having an irregular shape no uniform layer of fluorinated coating can be obtained.
According to another method of obtaining fluorinated coatings on non-fluorinated polyesters, a non-fluorinated polyester is coextruded with a fluorinated copolymer, for example a tetrafluoroethylene/hexafluoropropene polymer. This method, however, besides requiring a particularly complex technology, can be rarely used, as it can be applied only for particular types of articles, for example fibers.
By consequence, there was the requirement of having available readily processable polyesters which permit to overcome the above-said problems of finishing or of surface treatment, and at the same time having the above-cited characteristics of chemical inertia, mechanical properties, oil- and water-repellency, biocompatiblity.